PROPERTIES OF THE RESIN IMPREGNATED CORE PAPER ... (CFRP) as face sheets and non-metallic honeycombs as core material is applied due to.
SIMULATION OF THE DAMAGE TOLERANCE BEHAVIOUR OF CFRP/HONEYCOMB SANDWICH BASED ON MEASURED PROPERTIES OF THE RESIN IMPREGNATED CORE PAPER F. Hähnel, K. Wolf Dept. of Aerospace Engineering, Technische Universität Dresden, 01062 Dresden, Germany
KEYWORDS:
sandwich, honeycomb, resin impregnated paper, damage tolerance, numerical simulation ABSTRACT
Structural sandwich is widely used as a lightweight design solution for load-carrying components. In aircraft structures particularly sandwich using carbon fibre reinforced plastics (CFRP) as face sheets and non-metallic honeycombs as core material is applied due to features such as high strength-to-weight and stiffness-to-weight ratios as well as excellent fatigue behaviour. Owing to the rather weak core material, sandwich is prone to a range of defects and damages as a result of impact loading which may accidentally occur during assembly or operation of aircraft. These damages and their effect on the load carrying capability of the structure have to be considered in the damage tolerant design of aircraft. Therefore, it is necessary to determine the extent of damage in sandwich structures resulting from impact events and to predict the residual strength of the damaged components. Currently, in the development process this task is mainly done experimentally by using drop tests to simulate the impact loading, NDT methods to determine the damage size and compression or shear after impact (CAI, SAI) tests to investigate the residual load carrying capability of the damaged structures. These experimental procedures are rather costly and time consuming. Therefore, research has been done to develop reliable simulation procedures which are able to predict the damage tolerance behaviour based on a close to reality structural discretisation which requires knowledge of basic material properties of the sandwich constituents. As far as the global behaviour of sandwich components is investigated by finite element methods, it is sufficient to model the structure by using shell elements for the skins and solid elements for the core. In the case of honeycomb sandwich such models permit only a macromechanical description of the core behaviour. Thus, it is not possible to account for local failure modes of the hexagonal cell structures. Nevertheless, these local effects are important in cases, where the damage tolerance behaviour is of interest. For this kind of problem a detailed modelling of the honeycomb cell structure is required [1, 2, 3]. For example, impact loading of honeycomb sandwich results in very complex damage modes in the core, as can be seen in Figure 1a. In the centre of the impact area the honeycomb material is crushed. This damage is mainly the result of local buckling of the cell walls in combination with compressive failure of the cell wall material which is usually a resin-impregnated paper. Closer to the edge of the impact area the core is subjected to high shear forces which result in shear cracks in the cell walls. This clearly shows that material properties such as stiffness and strength of the honeycomb material are crucial parameters for the formation of damage in the sandwich core. Therefore, the knowledge of these properties is essential for a reliable numerical simulation of the damage tolerance behaviour of impact loaded honeycomb sandwich structures as shown in Figure 1b. Usually, honeycomb manufacturers provide material data only for honeycomb blocks instead of the impregnated papers used as core material. Hence, one approach is to determine the paper properties from the global core properties. This can be done by numerical simulation of the tests used to measure the global properties [1]. This approach provides only average data and it is difficult to identify the non-linear behaviour of the material. Thus, in case a more
thorough knowledge is required the material properties of the resin-impregnated paper have to de determined directly by appropriate test procedures [3, 4]. In the research presented in this paper a standard test method for paper and board [5] was used for the identification of tensile properties. The compressive mechanical properties of the different impregnated papers investigated were determined by utilizing a special test method (Figure 2a) which was improved compared to [3]. Further a special test fixture was developed for the determination of the behaviour of impregnated paper under shear loading (Figure 2b). For this test fixture the concept of a picture-frame was chosen and adapted to the requirements of the thin-walled material. These test procedures have been used to carry out an extensive test programme on different papers which were impregnated with phenol resin. Several parameters such as the paper thickness and the influence of the production process were investigated. One of the findings was that the investigated papers show a distinct orthotropic behaviour. Another basic result was that the stress-strain relation depends strongly on the resin fraction. (Figure 2c). Further results of this experimental study are presented in the paper. Also the influence of the paper properties on the numerical simulation of the damage tolerance behaviour will be given. a)
b)
Fig. 1: a) Impact damage of a non-metallic honeycomb sandwich structure; b) Numerical simulation of impact and compression after impact behaviour of honeycomb sandwich structure 600
b)
resin impregnated paper
Force [N]
load cell
10 mm
a)
c)
400 direction 0° 90° 0.60 phenol resin volume fraction 0.48 0.00
200
0 0,0
0,1
0,2
Shortening [mm]
0,3
Fig. 2: a) Compression test fixture; b) Picture-frame shear test fixture; c) Obtained material characteristics under compression loading for impregnated paper with different resin volume fraction
REFERENCES 1. Hähnel, F. Wolf, K. “Numerical Simulation of CFRP Honeycomb Sandwich subjected to Low-Velocity Impact”, Proceedings of Conference on Sandwich Construction 5, Zürich, Vol. 2, pp 657-666, 2000. 2. Nguyen, M. Q., Jacombs, S. S., Thomson, R. S., Hachenberg, D., Scott, M. L., “Simulation of impact on sandwich structures“, Journal of Composite Structures, Vol. 67, pp. 217-227, 2005 3. Hähnel, F., Wolf, K., “Evaluation of the Material Properties of resin-impregnated NOMEX® Paper as Basis for the Simulation of the Impact Behaviour of Honeycomb Sandwich“, Proceedings of Conference on Composite Testing and Material Identification, Porto, 2006 4. Foo, C. C., Chai, G. B., Seah, L. K., “Mechanical Properties of Nomex Material and Nomex Honeycomb Structure“, Journal of Composite Structures, Vol. 80, pp. 588594, 2007 5. ISO 1924-2:1994 en, Paper and board – Determination of tensile properties, Part 2: Constant rate of elongation method
